Perimeter weighting in iron-type golf clubs distributes non-essential mass of the iron towards the perimeter, reducing the effects that off-center hits have on the golf club and producing more accurate and consistent golf ball trajectories. Perimeter weighting is achieved by creating a cavity in the back of the golf club opposite the face or hitting surface. The material weight removed to create this cavity is redistributed around the perimeter of the golf club head. In general, larger cavity volumes correspond to increased amounts of mass distributed around the perimeter.
Removing material from the rear of the club head, however, reduces the thickness of the club face. Since the club face is the hitting surface, the club face cannot be so thin that the strength of the club face surface is not sufficient to withstand the stress resulting from a golf ball striking the club face. Reducing the thickness of the club face may also increase vibrations upon impact. These vibrations may cause bad feel to the user.
There are various examples of secondary material incorporation into iron golf club heads for vibration damping. Some of these materials provide the additional benefit of displacing weight to the perimeter of the club head so as to increase the club head's moment of inertia (MOI). These materials can provide a variety of different playing characteristics with the result that two clubs that are similar in appearance can in fact be different when played.
Because a playing characteristic of a golf club may not be evident from the shape of the club, the club may not perform the way a golfer expects it to. For example, a skilled golfer accustomed to playing with a club that has little or no damping material may unintentionally make a shot using a club that has a very forgiving damping material and find that the club performs unpredictably. Similarly, two clubs having the same shape may include damping material that provides very different weight distributions such that one of the clubs has a much lower center of gravity. A golfer who unwittingly switches one club for the other midway through a game may find that their next shot launches in an unexpected direction, worsening the golfer's score.